2.2. Sampling of Section No. 1
The upper part of Section No.
1, from 0 cm to ~12 cm (Fig. 2), which is called Bed No.
1 (Kadlec et al. 2014), is composed of fine backwater sediment of brown
clayey silt with white angular clasts of weathered limestone and bone
fragments (Bed No. 1) (Kadlec et al., 2014). The lower part of the
section, from ~12 to ~35 cm, consists of
brown silty clay without white clasts (Bed No. 2) (Kadlec et al., 2014).
Although our “Section No. 2” is the same as “Profile 2” of Kadlec et
al. (2005) and “Section No. 2” of Kadlec et al. (2014), our “Section
No. 1” is not the same as “Profile 1” of Kadlec et al. (2005) but it
is the same as “Section No. 1” of Kadlec et al. (2014). The depths in
the present study are not the same as those in Kadlec et al. (2005,
2014).
Our sample collection involved the larger sediment part below the
transition and the smaller part above the transition according to the
position of the reversed polarity in Kadlec et al. (2014). We chose
three sets of overlapping boxes (Fig. 2) to completely characterize the
transition. The 35.1 cm of exposed sediment at the base of Section No. 1
was flattened to a clean vertical face, and the samples for the
paleomagnetism measurements were taken by pushing the plastic boxes
(2x2x2 cm; 8 cc) into the sediment (Fig. 2). We used a Brunton
geological compass to measure the azimuth and tilt of the boxes. In
addition, another 4 samples were collected for rock magnetism
measurements (Fig. 2c), which corresponded with the paleomagnetic
samples (13_0P, 7_7P, 17_2M, 22_0M).
We examined the sediment structure near the walls of the plastic
sediment holder and observed that the process of pushing the box into
the sediment created deformation structures along the walls of the
boxes. The structure was on the order of 0.05 mm thick. With a box
volume of 8000 mm3 (20x20x20 mm) and a structurally
modified layer volume of 80 mm3 (4x 0.05x20x20 mm),
the volume modified by pushing is 1/100 of a fraction of the unmodified
volume. Even if this moment would be organized in (e.g., perpendicular
direction), it would only deflect the magnetic remanence by
<5%.